Effect of Nanobubbles in Electrolytically Generated Ozonated Water
Electrochemical ozone generation is a technology that enables in-situ ozone production from water, introducing ozone directly into the aqueous phase.
In certain systems, the design of the electrolytic cell and the hydraulic operating conditions may promote the formation of very small gas bubbles, increasing the gas–liquid contact surface and potentially improving ozone transfer into the water.
Why Can Ozonated Water Have Little or No Noticeable Ozone Odour?:
Odour should not be considered an appropriate method for determining ozone concentration in water or assessing the effectiveness of a water treatment process.
When ozone is generated and introduced directly into water, part of it may remain dissolved, while another part may react with substances present in the water or naturally decompose.
A system designed to promote efficient ozone transfer into water may reduce immediate ozone losses to the gas phase compared with systems producing larger bubbles. Therefore, a low perception of ozone odour does not necessarily mean that ozone is absent from the water.
The actual ozone concentration should be determined using an appropriate measurement method.
What Are Nanobubbles?:
The term nanobubbles is used to describe extremely small gas bubbles dispersed in a liquid. Their very small size provides a high surface-area-to-volume ratio and may result in different behaviour compared with conventional macroscopic bubbles.
Depending on the generation technology, water composition and hydraulic conditions, very small bubbles may remain dispersed in the liquid for longer periods than larger conventional bubbles.
To claim that a specific device generates nanobubbles, and particularly to specify their size distribution, experimental characterisation using appropriate measurement techniques is required.
Electrolytic Generation of Ozone Directly in Water:
In electrochemical ozone generation systems, ozone is produced in situ by means of an electrolytic cell.
One of the main characteristics of this technology is that ozone can be generated directly in contact with water, avoiding the need to first produce a gaseous ozone stream using a conventional ozone generator and subsequently transfer that gas into the liquid.
The final behaviour and concentration of ozone in the water depend on several factors, including:
- electrode technology and configuration;
- applied current density;
- water flow rate and pressure;
- temperature;
- pH;
- water conductivity and composition;
- ozone demand of the water;
- and contact time.
Ozone Transfer Using Very Small Bubbles
Reducing bubble size increases the interfacial area available between the gas and liquid phases.
Under suitable operating conditions, this may provide several technical advantages:
Increased gas–liquid contact. A larger specific surface area can facilitate mass transfer between the gaseous phase and the water.
Reduced immediate off-gassing. Smaller bubbles behave differently from larger conventional bubbles, which tend to rise rapidly to the surface and release gas into the surrounding atmosphere.
Improved distribution throughout the water. A fine bubble dispersion may contribute to a more homogeneous distribution of ozone within the hydraulic circuit.
Efficient use of ozone’s oxidising properties. Once introduced into the water, ozone can react with different oxidisable substances present in the medium.
Ozone and Oxidation Processes
Ozone is a powerful oxidising agent. Under certain conditions, its decomposition in water can also participate in reaction mechanisms involving reactive oxygen species, including hydroxyl radicals (•OH).
The extent and importance of these reactions depend considerably on the physicochemical characteristics of the water and the operating conditions.
Therefore, a specific production of hydroxyl radicals or a particular treatment efficiency should not be assumed solely from the presence of very small bubbles or nanobubbles.
Application in Water Treatment
Electrolytic ozone generation can be incorporated into systems designed for oxidative water treatment.
Where equipment is specifically intended for a biocidal purpose.
Technical Advantages of Electrolytic Ozone Generation
Depending on the system design and intended application, this technology may offer:
- in-situ ozone generation;
- direct introduction of ozone into the water circuit;
- compact system design;
- no requirement to store ozone;
- control of ozone production through electrical operating parameters;
- integration into different water-treatment circuits;
- and a high gas–liquid contact surface when the system produces a sufficiently fine bubble dispersion.
Safety and Control:
Ozone is a powerful oxidising agent and should be generated and used with equipment specifically designed for this purpose.
Ozone concentrations in water and, where applicable, in the surrounding atmosphere should remain within the limits and operating conditions established for the intended application.
The installation should be designed to minimise uncontrolled release of gaseous ozone and should incorporate the appropriate safety measures.
Conclusion:
The combination of electrochemical ozone generation and a fine dispersion of very small gas bubbles is an interesting technology for improving contact between ozone and water.
One of its main technical advantages is the ability to produce ozone in situ and introduce it directly into the aqueous medium.
The actual performance of the treatment should be assessed according to the ozone concentration achieved, water characteristics, contact time and the specific intended application.
When terms such as “ozone nanobubbles” are used to describe a particular system, the size and characteristics of those bubbles should be supported by appropriate technical characterisation and testing.